1. Field of the Invention
This invention relates to the field of fiber optic cable ducts for containing and protecting fiber optic cable, and more particularly concerns innerducts for protecting the cable and for reducing the friction drag on the fiber optic cable as it is being pulled through the innerducts to allow for longer and more efficient cable pulls.
2. Description of the Prior Art
Fiber optic cables are coming more and more into use in telephone lines, and they present problems of how to protect them from damage during installation and while installed in place. During installation in telephone networks, the fiber optics cable may be drawn through banks of 4 inch inside diameter ducts, with the ducts being made of concrete, clay tile, or polyvinyl chloride. For protection the fiber optic cable may be sheathed in polyethylene innerducts which are placed in the ducts and then the fiber optic cable is pulled through the innerducts. There may be 8 or 12 ducts placed side by side in a duct bank. Usually, no more than three or four innerducts may be placed in a duct.
In the prior art, innerducts had smooth walls, but when the fiber optic cable was pulled through the innerducts, there was so much frictional drag that the fibers of the cable were caused to separate. In general, if the frictional drag on the fiber optic cable exceeds about 600 pounds, the fibers separate.
Of course, the longer the cable pull, the cheaper the cable installation becomes because for a given distance you do not have to set up men and winches so often since you can set them up at distances which are farther apart.
In order to extend the length of the pull and reduce friction drag on the cable, various suggestions have been made. One suggestion was to provide corrugated duct having transverse ribs. However, this was not too successful since the corrugated ribs wiped the lubricant off the fiber optic cable.